I'm currently moving ahead on the proportional controls for a set of cooling vats. I'll be using Solo SL4824-CR temperature controllers to provide proportional-only control. Given that these tanks will be in close proximity, and in straight rows, I'll be connecting all the controllers with RS-485 and using a Modbus RTU to Modbus TCP gateway to read/write set value and read the process values.

Right now, I will need a Modbus RTU to Modbus TCP gateway to connect to a PLC and provide integration with a Glycol chiller. Does anyone have any suggestions on a solid, cost-effective device? This is my first time building a RS-485 network from scratch.

But don't get a general-purpose "serial server" with RS-485 on one side and Ethernet on the other. You need something that's built to understand both Modbus/TCP and Modbus RTU.

If you were using a PC and not a PLC, you might be able to use a general purpose serial server because the PC would be running a virtual serial port and could simply run a Modbus RTU driver directly. But a PLC is going to expect to use Modbus TCP, not just "encapsulated" Modbus RTU.

The several that I have used have been in service for at least 5 years and continue to work OK.

Several comments on their use:
1. The device comes as a Modbus version (as opposed to generic serial server), but Modbus firmware is not installed when you take it out of the box. It's on the CD or available as a download, but the firmware needs to be installed.

2. The module is DIN rail mount but does not have removeable screw terminals. If the device needs to be replaced, the wiring needs to be moved screw terminal by screw terminal.

3. If I recall properly, there's a Windows software installer package the assigns an IP address to the module and manages the Modbus firmware download to the module, and includes a telnet app that is used to configure the serial side of the gateway; very 1970-ish, but it works.

My advice is to make sure whichever one you work with, make sure that it is made for Modbus, so that it packs message frames on both sides properly.

Thanks for the info. I'm about to wrap up for the day. Gotta love laying out terminal blocks by hand

To be more specific:

Process Controller:
Handle Valve action with P control (2-3 degree + or - for a given set point)
Allow changing of the Setpoint at the station
4-20mA output for temperature (to reduce load on RS-485 Network)

PLC:
Write to setpoint and deadband from a parallel HMI and SCADA
Read the value of a flow switch
Provide the alarming
Integrate with the glycol chilling units

I'm not sure on the PLC yet, as I'm not that far into it. I was thinking about using a Click PLC to read the modbus values for alarming and to interface with the SCADA and HMI, and at first I will likely just connect the PLC to the RS-485 network, as it will only be used to start and stop the glycol chillers at the moment so that we can get him up and running by April.

Eventually I'll be pumping all this data into their process tracking software, hence the reason I want to put each group of tanks on a gateway.

If you use a Click with the Ethernet Port you could use the PLC as your Modbus RTU to Modbus TCP Gateway. They have a 485 port to communicate with the Controllers and a Ethernet Port to go to a HMI or SCADA System.

It would make the set up simpler.

__________________
There is never enough money to do it right but there is always enough money to do it again.

If you use a Click with the Ethernet Port you could use the PLC as your Modbus RTU to Modbus TCP Gateway. They have a 485 port to communicate with the Controllers and a Ethernet Port to go to a HMI or SCADA System.

It would make the set up simpler.

Not really, because you would need a way to retransmit messages between Click ports.

__________________It's not all the variables I am most concerned with, it's the undiscovered constants.

Not really, because you would need a way to retransmit messages between Click ports.

Maybe I am misunderstanding what the OP is wanting to do but, couldn't sparkie use the 485 port as a Modbus Master connected to the Temp. Controllers? The Serial Port would Write the Setpoint and Deadband to the controller and read other data that is required.

The Ethernet Port would then be a Modbus TCP/IP Server. The HMI/SCADA would send the Setpoint & Deadband to a PLC Modbus Register (which would then be sent out of the Serial Port). The HMI/SCADA could also read all the data from the PLC that is read from the Temp Controllers.

While not a Modbus Gateway per se. It may do just what the OP needs without the extra equipment.

If I am mistaken please let me know because I am always trying to learn.
Thanks,

__________________
There is never enough money to do it right but there is always enough money to do it again.

Maybe I am misunderstanding what the OP is wanting to do but, couldn't sparkie use the 485 port as a Modbus Master connected to the Temp. Controllers? The Serial Port would Write the Setpoint and Deadband to the controller and read other data that is required.

The Ethernet Port would then be a Modbus TCP/IP Server. The HMI/SCADA would send the Setpoint & Deadband to a PLC Modbus Register (which would then be sent out of the Serial Port). The HMI/SCADA could also read all the data from the PLC that is read from the Temp Controllers.

While not a Modbus Gateway per se. It may do just what the OP needs without the extra equipment.

If I am mistaken please let me know because I am always trying to learn.
Thanks,

Yes, that would work, but would probably require more time to set up than a generic gateway and would be more application specific. Changes to the addressing used would require changes in the Click too. If you knew you were only going to use those addresses and not modify it after it was all set up, it might not be any more work.

__________________It's not all the variables I am most concerned with, it's the undiscovered constants.

The reason I can't just use a Click as a modbus gateway is because these tanks will be grouped by 10's, and they will be adding tanks in off numbers, such as 3 or 4 at a time as they get them up and running and expand. So I feel the best route would be to keep each RS-485 network separate, and use a modbus gateway at each group of tanks. Then the PLC and SCADA systems can use eithernet. This enables me to significantly reduce the amount of wire that must be pulled to each panel. We can, for instance, use a single 20A circuit for power at each set of tanks and an ethernet wire to the gateway. Everything else is local to the tanks.

However, you are on to something. If I use a small click panel at each set of tanks, I have less distance to run instrument wire, and can use the click as the gateway. I would have to set up the polling times for info, and work a little handshaking magic, but that might be the best way to go.